First off, JBossAS v3 is seriously old (3.2.6 was released in October 2004). You're running a huge risk by continuing to use it, since you'll get absolutely no meaningful support from RedHat (or anyone else).

Having said that, it does contain a very early version of JBossCache, which probably has some basic support for replicated (i.e. "clustered") caches. There's very little documentation from back then, but it will resemble the standalone version of JBossCache that later became v1.4. See the docs to see where to start looking.